1. Field of the Invention
The present invention concerns a lifting apparatus of a type which comprises a base, a platform disposed above the base, thrust means therebetween to lift the platform relative to the base, and platform stabilizing means enclosed within the thrust means to connect the platform and base in parallel. The platform stabilizing means comprises an upper extensible linkage device having upper end portions which are connected to the platform, a lower extensible linkage device having lower end portions which are connected to the base, and connecting means which connect lower end portions of the upper extensible linkage device to upper end portions of the lower extensible linkage device, so that the linkage devices are constrained to move in unison.
2. Description of the Prior Art
Apparatus of the aforementioned type is known from U.S. Pat. No. 2,725,578 Keller dated Dec. 5, 1955. However, in this known apparatus, several of the linkage devices form parts of a vertically and horizontally extending bracing means, incorporating three separate lazytongs arranged at the sides of an equilateral triangle to provide vertical bracing in three mutually inclined vertical planes. Each of the lazy-tongs is slidably connected to the base and to the platform, and includes upper and lower linkage devices, as well as an intermediate linkage device which serves as a connecting means to connect the upper and lower linkage devices. A central pivot of each of the three upper linkage devices is connected to an upper peripheral circular ring member, and central pivots of the three intermediate and three lower linkages are similarly connected respectively to an intermediate ring member and a lower ring member. The three ring members are arranged horizontally to form part of the bracing means, and are secured to the inside of a pleated bellows which serves as the thrust means. In this known apparatus, the three surrounding ring members and the three separate lazy-tongs necessary for platform stabilization give rise to various problems, e.g. an extremely high manufacturing cost, complexity, and, most importantly, poor stabilization of the platform due to uneven or unequal movement (extension or contraction) of the lazy-tongs which allows the platform (and/or ring members) to tilt relative to the horizontal. Furthermore, the bearing or pivot clearances, which are necessary for relative pivotal movement of the parts of the lazy-tongs relative to each other and relative to the ring members, allow the platform to move linearly in any direction in or near to the plane of the platform. In any lifting position of the platform intermediate its maximum upper and minimum lower positions, the bracing means allows the platform to rotate freely through a restricted but unacceptably large angle, thus failing to stabilize the platform and allowing twisting torsion loads to be applied to the relatively weak bellows. Thus, the platform can pitch, roll and yaw to an unacceptable extent.
A further drawback of this known apparatus is that the ring members have to be directly attached to the bellows and subject the lazy-tongs to lifting forces which are produced by the bellows; thereby giving rise to the further problems of wear, friction and malfunction of the platform stabilizing means.
However, this known apparatus has the advantage that the horizontal dimensions of the linkage devices are relatively small in relation to the overall lift distance which can be produced by the apparatus, so that the apparatus is compact when the platform is lowered.
An object of the present invention is to enable the problems encountered with heretofore known lifting apparatus to be reduced or avoided.